Pakistani security and health officials said at least 31 people were killed and 35 wounded when a suicide bomber struck outside a crowded polling station in Quetta, the provincial capital of Balochistan Province.

Several people were reported to be in critical condition, raising concerns that the death toll could rise.

A local journalist who was at the scene when the blast struck said the explosion hit outside a polling station that was then closed and has since been reopened.

"Most of the dead and injured" were voters, said journalist Zainuddin, who goes by only one name.

The Islamic State (IS) extremist group claimed responsibility for the attack. The group said the attack was carried out by a suicide bomber, but did not provide further details or evidence for its claim.

Earlier this month, the Islamic State (IS) extremist group said it was behind a suicide attack that killed 149 people at an election rally in the town of Mastung in Balochistan.

More than 180 people, including three candidates, have been killed in campaign violence since June.

Earlier on July 25, an exchange of gunfire at a polling station in the Swabi district in northwestern Pakistan left one PTI party activist dead and four people injured.

Massive Military Deployment

At the request of the Election Commission, the military has deployed more than 370,000 personnel nationwide in and around polling stations -- the largest such deployment in Pakistan's history on an election day.

An additional 450,000 police officers have been assigned to provide security.

Polls show a tight race, with Khan's party ahead in one survey with 30 percent, compared to 27 percent for PML-N, the outgoing ruling party. Sharif's party leads another poll with 26 percent compared to 25 percent for Khan's PTI.

In third place is the Pakistan People's Party (PPP) of Bilawal Bhutto Zardari, 29, the son of former Prime Minister Benazir Bhutto, who was assassinated in 2007. The PPP could emerge as a coalition kingmaker if no party wins a majority, as many experts expect.

"Our predictions are very murky right now," said Bilal Gilani, executive director of Gallup Pakistan, adding that a huge chunk of voters remain undecided. "It's still up for grabs."

Khan cast his ballot near his suburban home in the capital, Islamabad, appealing to Pakistanis to come out in large numbers and vote "to save future generations."

Meanwhile, Zardari cast his vote in his native town of Larkana in Sindh Province, while PML-N leader Shahbaz Sharif, brother of the jailed former prime minister, voted in the eastern city of Lahore.

Asif Khan, a voter in Lahore, said it was "chaotic" inside the polling station where he cast his ballot but said soldiers and police were maintaining security.

"I'm not casting my vote for a particular party," said Iqbal Hussain, a voter in the Swat Valley in northwestern Pakistan. "Rather I am using my vote for Pakistan. I also want to elect a representative to solve our problems relating to roads, water and education."

Syed Sardar Ali, a voter in the city of Rawalpindi, said he was voting to "bring some change" in Pakistan.

The most pressing issues for voters are corruption, the military's influence in political affairs, energy shortages, and infrastructure development.

Almost 106 million voters are eligible to cast ballots for the 342-member National Assembly, or the lower house of parliament, and assemblies in Khyber Pakhtunkhwa, Punjab, Sindh, and Balochistan provinces.

Sixty seats in the National Assembly are reserved specifically for women, and 10 for non-Muslim minorities.

Long Lines

The 85,307 polling stations opened at 8 a.m. local time and are due to close at 6 p.m

Preliminary results are expected to trickle in by late July 25. A clearer picture of the outcome is expected in the early hours of July 26. Preliminary final results are expected later that day.

The PML-N has made a request for polling stations to stay open for another hour due to long lines.

Local television reported sporadic cases of police arresting voters with pre-marked ballots.

Michael Gahler, the head of the European Union's election monitoring mission in Pakistan, said he was looking into reports that media access was restricted by soldiers posted at polling stations.

Gahler will present a preliminary report on July 27 assessing the elections.

Another concern surrounding the vote is the unprecedented number of extremist religious parties that are participating in the election, including some that have been banned but are running under new names.

Local television showed U.S.-designated terrorist Hafiz Saeed, who has a $10 million bounty on his head, casting his ballot in Lahore

The vote marks only the second time in Pakistan’s 70-year history that a civilian government has completed a full term and handed over to another civilian administration through the ballot box.

Pakistan's military has ruled for approximately half the period since the country’s independence in 1947, staging coups three times.

'Blatant' Manipulation

The campaign has been characterized by "blatant, aggressive and unabashed attempts to manipulate" the outcome, with a crackdown on the media and the intimidation of candidates, according to Pakistan's Independent Human Rights Commission.

At the center of most allegations is the powerful military establishment, which has an oversized role in the country's domestic and foreign affairs.

The PML-N has alleged pre-poll rigging and accused the army of influencing the judiciary to deny it a second term.

Former premier Sharif was sentenced to 10 years in prison on corruption charges in absentia. He was arrested after returning to Pakistan on July 13 and has appealed his sentence.

Sharif, a vocal critic of the army, was dismissed from office by the Supreme Court in July 2017 for allegedly concealing assets abroad and other corruption allegations. He denies any wrongdoing.

Allies of the three-time prime minister, whose previous term ended when he was toppled in a military coup in 1999, have called the proceedings against him a political vendetta and suggested the army might be behind it.

Sharif has accused the army of paving the way for Khan, which the former cricket star denies.

The campaign also has been marked by violence, with a string of militant attacks killing more than 180 people, including three candidates.

The military has stationed over 370,000 personnel nationwide to ensure the vote goes smoothly -- the largest such deployment in Pakistan's history on an election day. An additional 450,000 police have also been assigned to provide security.

An unprecedented number of extremist religious parties are also participating in the election, including some that have been banned but are running under new names.

Political parties held their final rallies on July 23 before campaigning was suspended.

"I am giving this task to all of you to wake up early on the 25th and cast your vote," Khan told supporters in Lahore.

Shahbaz Sharif said victory was "certain."

"Despite all the odds PML-N is winning the July 25 polls," he was quoted as saying by Pakistani media.